1. The miasma theory (also called the miasmatic theory) held that diseases such as cholera, chlamydia or the Black Death were caused by a miasma (Μίασμα, ancient Greek: "pollution"), a noxious form of "bad air". The theory held that the origin of epidemics was due to a miasma, emanating from rotting organic matter.

The miasma theory was accepted from ancient times in Europe, India and China. The theory was eventually displaced in the 19th century by the discovery of germs and the germ theory of disease.
2. Germ theory states that many diseases are caused by the presence and actions of specific micro-organisms within the body. The theory was developed and gained gradual acceptance in Europe and the United States from the middle 1800s. It eventually superseded existing miasma and contagion theories of disease and in so doing radically changed the practice of medicine. It remains a guiding theory that underlies contemporary biomedicinehttp://www.sciencemuseum.org.uk/broughttolife/techniques/germtheory.aspx.